Modern oil field operations demand a great quantity of information relating to downhole conditions. Such information typically includes characteristics of the earth formations traversed by the borehole, data relating to the size of the borehole, and data relating to the configuration of the borehole. The collection of information relating to downhole conditions, which is commonly referred to as logging, can be performed using several methods including wireline logging and logging while drilling (LWD). A variety of logging tools are available for use with each of these methods.
One example of a wireline logging and LWD tool is a high-frequency dielectric tool (HFDT). A HFDT determines the dielectric constant and conductivity of downhole formations from the real and complex parts of the propagation constant of electromagnetic waves traveling through the formations. Considering a specific example, a HFDT may use a microwave signal of 1 gigahertz to measure the bulk volume of water (BVW), which helps to evaluate the amount of movable hydrocarbons in a reservoir.
The signals received by a HFDT are subject to attenuation of 50 dB or more. As such, increasing signal strength, while at the same time reducing noise and unintended couplings, is paramount. One way to increase signal strength is through impedance matching.
A HFDT typically includes multiple resonant cavity antennas, each coupled to a coaxial feed cable. As a signal travels through, out of, and back into the HFDT via the transmit and receive antennas it encounters differences in impedance especially at interfaces, e.g. from formation to antenna. At each interface, a portion of the signal's energy reflects back depending on how well the impedances are matched. The less energy that is reflected, the better the impedance is matched and the more power transfers through the interface. In conventional dielectric logging systems, the impedance matching is implemented by connecting the receive cavity antenna directly to the coaxial cable, which may have an impedance of 50Ω or 75Ω. While simple, this method results in a weak average signal because the impedance matching is not robust.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.